The present invention relates to servo systems in disc drives. In particular, the present invention relates to compensation for errors in servo systems.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable runout errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable runout errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in an inner track path that interferes with an outer track path. This is especially acute when a first written-in error causes a head to be outside of an inner track""s ideal circular path and a second written-in error causes the head to be inside of an outer track""s ideal circular path. To avoid limitations on the track pitch, a system is needed to compensate for repeatable runout errors.
The written-in errors can be compensated using stored compensation values which are used as additional inputs, into the servo loop. However, sufficient memory is required to store all of the compensation values.
The present invention addresses these and other problems, and offers other advantages over the prior art.
The present invention relates to disc drives that have compensation for repeatable runout errors which address the above mentioned problems.
In accordance with one embodiment of the invention, a disc drive having a servo loop for positioning a head over a disc. The servo loop includes a servo controller which generates a servo control signal in response to a received position error signal and an actuator, coupled to the servo controller is capable of moving the head in response to the servo control signal. A sensor, located in the head, is capable of sensing servo information located on the disc and producing a servo signal therefrom. The servo signal is combined with a reference signal to produce the position error signal. A compensation storage device stores decimated compensation values and a servo compensator is adapted to expand the decimated compensated value and inject the expanded compensation value into the servo loop.